According to a new study using Norwegian data, older siblings are 3 IQ points smarter than their younger brothers.

Older and Wiser?Â Birth Order and IQ of Young MenÂ
Sandra E. Black, Paul J. Devereux, Kjell G. SalvanesÂ
While recent research finds strong evidence that birth order affects children’s outcomes such as education and earnings, the evidence on the effects of birth order on IQ is decidedly mixed.Â This paper uses a large dataset on the population of Norway that allows us to precisely measure birth order effects on IQ using both cross-sectional and within-family methods.Â Importantly, irrespective of method, we find a strong and significant effect of birth order on IQ, and our results suggest that earlier born children have higher IQs.Â Our preferred estimates suggest differences between first-borns and second-borns of about one fifth of a standard deviation or approximately 3 IQ points.Â Despite these large average effects, birth order only explains about 3% of the within-family variance of IQ.Â When we control for birth endowments, the estimated birth order effects increase.Â Thus, our analysis suggests that birth order effects are not biologically determined.Â Also, there is no evidence that birth order effects occur because later-born children are more affected by family breakdown.

To the extent that IQ is a function of parental time, this makes intuitive sense to me.Â My wife and IÂ have a four month old son, and I can’t possibly imagine how we would ever give his sibling the amount of undivided attention that we lavish on him. Admittedly, we’ll be more experienced parents if/when the next one comes, but I expect the effect of the time constraint would dominate.

6 Responses to Go ask your older brother – he's the smart one

I haven’t read that article, but I’m sure there is others analyses out there that use number of children as a covariate (I can’t remember them right now — quite possible with older versions of the same data set). These studies show that the birth order effect is actually an N effect of number of children — less intelligent people tend to have more children, which looks like a birth order effect when you take the means in cross-sectional designs and don’t control for family size (the Norwegian data set allows you to do this). I’m going to guess that this is partially responsible for the tiny within family effect.

Conrad, this study beats previous ones due to its sample size, which solves the problem you mention. When you have IQ scores for all men in Norway, you can easily restrict the sample to 2-child families.

I’m sure some of the previous studies were actually done with that data set now I think about it — stretching my memory further, it was also used to investigate the “when to stop having children hypothesis” (i.e., when you have a boy and can’t tolerate the noise of any more children — I think the results were mixed) and some other rather amusing things. I presume this is the latest update and the more amusing analyses will come later.

More seriously, it would be very interesting to compare these results from countries with less extensive child-care than Norway (or look at working parents etc.) and see how much difference it makes, although I don’t know any other country that has that sort of data.

Andrew – of course the benefits of undivided parental attention are also affected by the timing of subsequent births. While I’m the eldest of my parents’ large family (and definitely the smartest :-) ), I was the only apple of their eyes for a short 12 months until my next sibling came along. I think that’s why some people who care about these things recommend having children at least 5 years apart if you want them all to be equally clever. That gets a bit difficult if you only start childbearing in your late 30s or early 40s of course.